Abstract: Instead of developing nitrogen prescriptions prior to the growing season, an in season site specific nitrogen management system was proposed which only a fraction of the normal nitrogen rates was applied near emergence, and additional nitrogen was applied only if and where the crop could utilize it. This paper outlines our efforts to develop two essential technologies for the proposed system: nitrogen sensing technology and variable rate application technology. Our sensor development work focused on optical remote sensing technology using multi spectral imaging sensors. Strong correlations between multi spectral images and chlorophyll meter readings were found, which indicated the great potential in using the imaging sensors to characterize corn nitrogen status (stress) on a whole field basis. In addition, a variable rate sprayer, consisted of PWM solenoids, a pressure controller, a nozzle control system interfaced to a computer, and map based application software, was also developed. The variable rate sprayer was able to provide independent rate control of 25 individual nozzles across an 18 m sprayer boom. Results from several field studies conducted in Iowa and Illinois were also presented. In 1998, the corn yield increased by an average of 1 440 kg/hm² in four study fields in both Illinois and Iowa. In 1999, the average corn yield increased by 440 kg/hm² in six study fields in Iowa.